The present invention relates generally to the field of process of forming metallic objects through directional solidification and more particularly to an optimized mold design for use in such process.
The process of directional solidification is well known in the production and manufacture of turbine elements such as rotor blades, stator vanes as well as other airfoil components having a single crystal structure. In general, the process involves providing a seed cavity at the base of the mold, wherein the restrictive shape of the pathway from the seed cavity to the part-defining cavity (sometimes referred to as the sorter or selector) encourages the formation of a single crystal structure during solidification of a molten alloy. The mold is then withdrawn from a heated environment or otherwise linearly cooled such that the propagating single crystal structure proceeds up the length of the part, thereby resulting in the entire part having this structure. Examples of conventional directional solidification processes may be found in U.S. Pat. Nos. 3,494,709, 4,940,073 and 5,062,469.
Suitable molds for use in such directional solidification processes traditionally include ceramic molds formed using the investment or “lost wax method” wherein a wax pattern having the desired shape of the mold is first formed. The wax pattern is then covered with a layer of ceramic material, from which the wax is then removed, thereby leaving a ceramic mold of the desired shape.
Unfortunately, conventional mold designs for producing gas turbine airfoils have failed to adequately provide for the substantial thermal stresses generated between the solidifying metal and the mold, resulting from inherent mismatches in thermal expansion coefficients. More specifically, exceptionally large solidification induced thermal stresses typically occur in the area between the sorter/seed assembly and the part-defining cavity. These result of such stresses often leads to premature mold failure or surface damage to the cost components.
Known efforts to address thermal stress build-up include reducing the mold withdrawal rate to thereby the build-up of such stresses. Unfortunately, this approach necessarily increases furnace cycle and overall manufacturing times. Additionally, the above approach can also result in increased metal/mold interaction times, mold bulging, and grain nucleation, each of which is disadvantageous.
Another known approach to thermal stress build-up involves the use of alternative mold materials which are better able to withstand the stresses. Unfortunately, such alternative materials introduce additional costs and complexity to the manufacturing process.
Accordingly, there is a need in the art of directional solidification for a mold design which optimally reduces the effects of thermal stresses, thereby enhancing mold life and the quality of components cast in the mold.